1. Field of the Invention
This invention relates to UV (ultraviolet) radiation curable compositions. More particularly, the invention relates to ultraviolet radiation curable compositions comprising epoxy functional compounds, silanol functional compounds, a photoinitiator and/or photosensitizer for an ultraviolet activated cationic cure, and preferably a carbonol functional polysiloxane.
2. Background Information
Epoxy resins find wide use industrially in many applications. Recent developments have provided the capability to UV cure epoxy resins via a photoinitiated cationic cure. Some typical cationic photoinitiators include aryldiazonium compounds, diaryliodonium compounds, triarylselenonium compounds, and triarylsulfonium compounds. The sulfonium compounds have found the most widespread use. Crivello in U.S. Pat. No. 4,058,401, issued Nov. 15, 1977; in U.S. Pat. No. 4,138,255, issued Feb. 6, 1979; and in U.S. Pat. No. 4,161,478, issued July 17, 1979, teaches the cationic polymerization of epoxy resin materials, such as epoxy monomers or prepolymers, by the use of certain radiation sensitive aromatic onium salts of Group VIa elements. Such onium salts of Group VIa elements have an MF.sub.6 anion where M is P, As, or Sb. These onium salts are found to be photoactive under ultraviolet light. Crivello teaches that curable compositions can be used as sealants, coating compounds, and encapsulants.
Smith in U.S. Pat. No. 4,256,828, issued Mar. 17, 1981, teaches photocopolymerizable compositions containing epoxides, organic material with hydroxyl functionality, and a photosensitive aromatic sulfonium or iodonium salt of a halogen-containing complex ion. Smith uses these compositions to coat substrates.
Eckberg and LaRochelle in U.S. Pat. No. 4,421,904, issued Dec. 20, 1983, teach combining epoxy functional diorganosiloxane fluids with bis-aryl iodonium salts, particularly linear alkylate bis-dodecylphenyl iodonium salts to form silicone coating compositions to provide non-adherent surfaces. Eckberg et al teaches that the adhesion of the silicone coating to a substrate can be improved with the addition of beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.
Pasternack et al in U.S. Pat. No. 4,585,534, issued Apr. 29, 1986, teach protecting a freshly drawn optical glass fiber with an ultraviolet-initiated, cationically curable liquid coating composition comprising a cationically curable polyepoxide, a polysiloxane carrying a plurality of hydroxyalkyl groups, and a photoinitiator and/or photosensitizer for an ultraviolet-activated cationic cure. Pasternack et al teach that the coatings have a low modulus and retain a low modulus at temperatures as low as -60.degree. C.
The introduction of carbonol functional silicone surfactants into epoxy resin to improve the flexibility, especially at low temperatures is described in EP Appln. 124,057, filed Nov. 7, 1984, by Pasternack et al as glass coatings comprising a cationically curable polyepoxide, a polysiloxane carrying a plurality of hydroxyalkyl groups and a photoinitiator. Further improvement in the cured epoxy resin properties such as additional flexibility, solvent resistance, thermal resistance, and improved moisture resistance are still needed for many applications, such as in electronics. Improved cure rate is also desirable. The cure rate is too slow for many applications and epoxy resins exhibit too much shrinkage during curing for many applications. The flow characteristics are not adequate for some coating applications such as in circuit board coating and paper coatings without the use of modifiers, including solvents. The epoxy resins are often too brittle for many coating uses to provide the desired protective properties. Thermal cure of epoxy resins also limits the utility of these materials to uses where the substrates can withstand the required temperatures of curing. It is these problems to which this invention is directed.